FERRITIC STAINLESS STEEL SHEET

Abstract

A ferritic stainless steel sheet includes a base metal and a nitrided layer that forms on a surface of the base metal. The chemical composition of the base metal contains, in mass percent, C: 0.001 to 0.020%, Si: 0.01 to 1.50%, Mn: 0.01 to 1.50%, P: 0.010 to 0.050%, S: 0.0001 to 0.010%, Cr: 16.0 to 25.0%, N: 0.001 to 0.030%, Ti: 0.01 to 0.30%, and optional elements, with the remainder being Fe and unavoidable impurities. The steel microstructure of the base metal includes, by volume proportion, 95% or more of a ferritic phase. The nitrided layer is a layer that is present in a region extending from a surface of a rolled product to a position of 0.05 µm depth in one direction of sheet thickness, and an average nitrogen concentration in the nitrided layer is, in % by mass, 0.80% or more.

Description

FERRITE STAINLESS STEEL SHEET FIELD OF INVENTION [1] The present invention relates to a ferritic stainless steel sheet. STATE OF THE ART [2] Automotive components include various types of parts and members such as an exhaust manifold, muffler, catalytic converter, flexible pipe, and center pipe. These components are repeatedly heated and cooled, so a ferritic stainless steel sheet is used for them, which resists thermal expansion and is suitable for heat-resistant applications. [3] A sheet of ferritic stainless steel used for the components described above is required to have heat-resistant properties and, more recently, has been required to have resistance to initial corrosion on an external surface of a member, in addition to heat-resistant properties. Here, initial corrosion refers to the red rust that occurs on a component and member that is relatively easily visible, such as an exhaust manifold and muffler, within a very short period of time from the time a car is shipped until before or immediately after use. Initial corrosion has no effect on the service life of a member, but it is undesirable in appearance. Thus, there is a demand to prevent or reduce the occurrence of initial corrosion. [4] For example, Patent Document 1 discloses an automotive exhaust component made of a steel having the same chemical composition as SUS 409L as a starting material. The automotive exhaust component is improved in its resistance to initial corrosion. [5] Furthermore, the automotive exhaust component is made to contain Cr, which is effective for corrosion resistance, specifically initial corrosion resistance, with a Cr content of 10.0 to 13.5%. In addition, initial corrosion resistance is enhanced by the formation of a coating film containing alkali or alkaline earth metal silicates on the component surface that will be exposed to the external environment. LIST OF RELATED TECHNICAL DOCUMENTS PATENT DOCUMENTS [6] Patent Document 1: JP2005-320559A SUMMARY TECHNICAL PROBLEM rcnonn / zznz / E / YiAi [7] The ferritic stainless steel sheet disclosed in Patent Document 1 requires a coating treatment on its surface to prevent or reduce the occurrence of initial corrosion. This raises the problem of increasing the number of processes and increasing production costs. [8] An objective of the present invention is to solve the problem and provide a ferritic stainless steel sheet for which the number of processes is reduced and which is able to prevent or reduce initial corrosion. SOLUTION TO THE PROBLEM [9] The present invention is made to solve the problem described above, and the essence of the present invention is the following ferritic stainless steel sheet.

[10] (1) A ferritic stainless steel sheet comprising a base metal and a nitrided layer forming on a surface of the base metal, wherein the chemical composition of the base metal consists, in % by mass, of C: 0.001 to 0.020%, See: 0.01 to 1.50%, Mn: 0.01 to 1.50%, P: 0.010 to 0.050%, S: 0.0001 to 0.010%, Cr: 16.0 to 25.0%, N: 0.001 to 0.030%, Water: 0.01 to 0.30%, Nb: 0 to 0.80%, Sn: 0 to 0.50%, Al: 0 to 3.0%, Day: 0 to 2.0%, V:0a 1.0%, Cu: 0 to 2.0%, Mo: 0a3.0%, Ca: 0 to 0.0030%, High: 0a0.1%, B: 0 to 0.0050%, W: 0 to 3.0%, Co: 0 to 0.50%, Sb: 0 to 0.50%, Mg: 0 to 0.0100%, rcnonn / zznz / E / YiAi Zr: 0 to 0.30%, Is: 0 to 0.10%, y REM: 0 to 0.05%, the remainder is Fe and unavoidable impurities, a steel microstructure of the base metal includes, by volume proportion, 95% or more of a ferritic phase, the nitrided layer is a layer that is present in a region ranging from the surface of a rolled sheet to a depth of 0.05 pm in the sheet thickness direction and an average nitrogen concentration in the nitrided layer is, by mass %, 0.80% or more.

[11] (2) The ferritic stainless steel sheet conforming to (1) above, wherein a chemical composition of the base metal contains one or more elements selected from, in % by mass: Nb: 0.10 to 0.80%, Sn: 0.01 to 0.50%, Al: 0.003 to 3.0%, Ni: 0.1 to 2.0%, V: 0.05 to 1.0%, Cu: 0.1 to 2.0%, Mo: 0.10 to 3.0%, Ca: 0.0001 to 0.0030%, and Ga: 0.0002 to 0.1%.

[12] (3) Ferritic stainless steel sheet conforming to (1) or (2) above, wherein a chemical composition of the base metal contains one or more elements selected from, in % by mass: B: 0.0002 to 0.0050%, W: 0.1 to 3.0%, Co: 0.02 to 0.50%, and Sb: 0.01 to 0.50%.

[13] (4) Ferritic stainless steel sheet conforming to any of points (1) to (3) above, wherein a chemical composition of the base metal contains one or more elements selected from, in % by mass: Mg: 0.0002 to 0.0100%, Zr: 0.05 to 0.30%, Ta: 0.01 to 0.10%, and REM: 0.001 to 0.05% rcnonn / zznz / E / YiAi ADVANTAGEOUS EFFECTS

[14] According to the present invention, it is possible to provide a ferritic stainless steel sheet for which the number of processes can be reduced and which can prevent or reduce initial corrosion. BRIEF DESCRIPTION OF THE DRAWINGS

[15] Figure 1 is a graph illustrating an example of nitrogen concentration distribution on a steel sheet surface in the direction of the sheet thickness depth. Figure 2 is a graph illustrating a relationship between the average nitrogen concentrations of the nitrided layers of steel sheets and the number of cycles in which pitting occurs. DESCRIPTION OF THE MODALITIES

[16] The present inventors carried out detailed studies on a ferritic stainless steel sheet that can prevent or reduce initial corrosion and obtained the following findings (a) to (d).

[17] (a) Initial corrosion is corrosion that forms on a surface, so surface treatments, such as coating treatments, are effective. Therefore, the present inventors paid attention, among surface treatments, to annealing nitriding, in which the annealing is carried out in a non-oxidizing atmosphere containing nitrogen gas and the like, from the point of view of reducing the number of processes and reducing production costs.

[18] (b) The present inventors considered that, by performing said annealing nitriding treatment, a nitrided layer is formed in which nitrogen is concentrated on a surface of a steel sheet, so that the initial corrosion resistance can be improved. However, under certain conditions for the annealing nitriding treatment and with certain chemical compositions of steel, performing the nitriding treatment can actually decrease the initial corrosion resistance and, moreover, result in poor material quality. This is attributable to the occurrence of sensitization or the formation of a martensitic phase.

[19] (c) The present inventors have taken note of the fact that adjusting the chemical composition and controlling the nitriding treatment conditions are effective in improving initial corrosion resistance. Preferred nitriding treatment conditions include preparing a non-oxidizing atmosphere consisting of 80 to 99% nitrogen gas and the remainder hydrogen gas, and performing annealing within the temperature range of 850 to 1000 °C.

[20] (d) Establishing an average nitrogen concentration from a surface of a steel sheet to a position 0.05 pm in one thickness direction of the sheet, i.e., in the vicinity of the surface of the steel sheet, at 0.80% or more under the conditions, results in a ferritic stainless steel sheet that has good resistance to early corrosion. Furthermore, in a case where the average nitrogen concentration is 1.0% or more, a ferritic stainless steel sheet with even better resistance to early corrosion can be obtained. rcnonn / zznz / E / YiAi

[21] The present invention is based on the findings described above. The requirements of the present invention will be described in detail below.

[22] 1. Configuration of the ferritic stainless steel sheet according to the present invention A ferritic stainless steel sheet according to the present invention includes a base metal and a nitrided layer that forms on a surface of the base metal.

[23] 2. Chemical composition of the base metal The reasons for limiting the content of each element in the chemical composition of the base metal are as follows. In the following description, the symbol % for each content means percent by mass.

[24] C: 0.001 to 0.020%. Carbon (C) degrades toughness, corrosion resistance (initial corrosion resistance), and oxidation resistance, so minimizing the C content is preferable. Therefore, the C content is set at 0.020% or less, preferably 0.010% or less. However, excessive reduction of C leads to increased refining costs. Therefore, the C content is set at 0.001% or more. Taking into account production costs and corrosion resistance, the C content is preferably 0.002% or more, and more preferably 0.005% or more.

[25] Si: 0.01 to 1.50%. Silicon (Si) is a deoxidizing element, as well as an element that improves corrosion resistance (initial corrosion resistance), oxidation resistance, and mechanical strength at high temperatures. Therefore, the Si content is set at 0.01% or more. Note that, to obtain a significant advantageous effect in improving corrosion resistance, the Si content is preferably 0.15% or more, more preferably 0.30%, and even more preferably 0.80% or more.

[26] On the other hand, a Si content above 1.50% makes the steel sheet significantly hard, which reduces the bending capacity of the resulting tube during machining. Therefore, the Si content is set at 1.50% or less. Considering toughness and pickling properties in the production of the steel sheet, the Si content is preferably 1.20% or less. A Si content of 1.00% or less is more preferable.

[27] Mn: 0.01 to 1.50%. Manganese (Mn) forms MnCr₂U₄ or MnO at high temperatures, improving scale adhesion. Therefore, the Mn content is set at 0.01% or more. The Mn content is preferably 0.15% or more, and more preferably 0.20% or more. However, if the Mn content exceeds 1.50%, corrosion resistance, particularly initial corrosion resistance, decreases, and the amount of oxides increases, which tends to lead to unusual oxidation. Therefore, the Mn content is set at 1.50% or less. Furthermore, considering toughness and pickling properties in steel sheet production, the Mn content is preferably 1.00% or less, and more preferably 0.70% or less. Furthermore, in the event that plate cracking attributable to oxides in a weld zone is taken into account, the Mn content is preferably 0.30% or less.

[28] P: 0.010 to 0.050%. Like silicon, phosphorus (P) is a solid solution hardening element, so its content is preferentially reduced to maintain material quality and toughness. Therefore, the P content is typically set at 0.050% or less. However, excessive P reduction leads to increased refining costs. Therefore, the P content is also set at 0.010% or more. Considering production costs and oxidation resistance, the P content is preferably 0.015% or more, and more preferably 0.030% or less.

[29] S: 0.0001 to 0.010%. Sulfur (S) content is preferably minimized from the standpoint of material quality, corrosion resistance (initial corrosion resistance), and oxidation resistance. In particular, if S is present in excess, it forms compounds with titanium (Ti) or manganese (Mn), causing cracks to originate from inclusions when the resulting tube is bent. Therefore, the S content is set at 0.010% or less. However, excessive S reduction leads to increased refining costs. Therefore, the S content is set at 0.0001% or more. Furthermore, considering production costs and corrosion resistance, the S content is preferably 0.0005% or more, and more preferably 0.0050% or less.

[30] Cr: 16.0 to 25.0%. Chromium (Cr) is an element that improves corrosion resistance (initial corrosion resistance) and oxidation resistance. To obtain sufficient corrosion resistance to prevent initial corrosion, the Cr content is set at 16.0% or more. The Cr content is preferably 16.5% or more, and more preferably 17.0% or more. However, if the Cr content exceeds 25.0%, toughness and producibility decrease. Therefore, the Cr content is set at 25.0% or less. The Cr content is preferably 23.0% or less. From a production cost standpoint, the Cr content is preferably less than 22.0%. Furthermore, from the standpoint of the toughness of a hot-rolled sheet in steel sheet production, the Cr content is preferably 18.0% or less.

[31] N: 0.001 to 0.030%. As with carbon, nitrogen (N) decreases low-temperature toughness and workability, and also reduces corrosion resistance (initial corrosion resistance) when rcnonn / zznz / E / YiAi combines with chromium to form a chromium nitride. Therefore, it is preferable to minimize the N content in the base phase of the steel sheet. The N content is thus set at 0.030% or less. The N content is preferably 0.020% or less. On the other hand, excessive reduction of N leads to increased refining costs. Therefore, the N content is set at 0.001% or more. Considering production costs and toughness, the N content is preferably 0.005% or more, and more preferably 0.008% or more.

[32] Ti: 0.01 to 0.30%. Titanium (Ti) enhances corrosion resistance (initial corrosion resistance), intergranular corrosion resistance, and deep-drawing capacity when combined with carbon, nitrogen, and sulfur. Furthermore, titanium nitrides increase the proportion of equiaxed crystals by acting as grain nuclei in continuous plate casting. This eliminates the coarse microstructure of steel resulting from columnar crystals, which causes surface irregularities, and improves surface quality.

[33] This immobilizing effect of C, N, and S through combination with these elements occurs when the Ti content is 0.01% or more. Therefore, the Ti content is set at 0.01% or more, and preferably at 0.11% or more. However, if the Ti content exceeds 0.30%, the dissolved Ti hardens the steel sheet and also decreases its toughness. Therefore, the Ti content is set at 0.30% or less. Taking into account production costs and the like, the Ti content is preferably 0.05% or more, and preferably 0.25% or less.

[34] In the present invention, it is preferable to contain, in addition to the chemical composition described above, one or more groups selected from the components of the following group A, group B, and group C, as required. Note that the elements classified as group A are elements that improve corrosion resistance, the elements classified as group B are elements that improve high-temperature properties, such as high-temperature mechanical strength, and the elements classified as group C are elements that influence toughness or surface texture.

[35] <Elementos del grupo A> Nb: from 0 to 0.80%. Like titanium, niobium (Nb) enhances corrosion resistance (initial corrosion resistance), intergranular corrosion resistance, and deep drawing capability when combined with carbon, nitrogen, and sulfur. Furthermore, niobium exhibits high solid solution hardening capacity over a wide temperature range, precipitation hardening capabilities, and also improves high-temperature strength and thermal fatigue properties. Therefore, its inclusion can be considered as needed. rcnonn / zznz / E / YiAi

[36] However, if Nb is present in excess, the toughness at a steel sheet production stage decreases significantly. Furthermore, during annealing, coarse carbonitrides or coarse intermetallic compounds known as the Laves phase precipitate. These precipitates establish grain boundaries to prevent recrystallization. As a result, there is a possibility of unrecrystallized structures remaining in the steel, which degrades surface quality. Therefore, the Nb content is set at 0.80% or less. The Nb content is preferably 0.55% or less. On the other hand, to achieve the desired effects, the Nb content is preferably 0.10% or more. Taking into account the intergranular corrosivity of a weld zone, production costs, and producibility, the Nb content is preferably 0.15% or more, and more preferably 0.30% or less.

[37] In this case, the total Ti and Nb content preferably satisfies the following formula (i). This is because if the total Ti and Nb content is less than 3(C+N), the C and N cannot be sufficiently fixed, and an excess of C and N may dissolve in the steel, making it hard and decreasing its workability. Nb + Ti > 3(C+N) (i) where the element symbols in Formula (i) above indicate the content (% by mass) of the elements contained in the steel, and when an element is not contained, zero will be put in the corresponding symbol.

[38] Note that to achieve the effect of increasing the proportion of equiaxed crystals in a cast steel microstructure, so as to eliminate the coarse steel microstructure derived from columnar crystals, the value on the left-hand side of Formula (i) above is preferably 0.10 or more, and more preferably 0.15 or more. Furthermore, from the standpoint of material hardness and production costs, the value on the left-hand side of Formula (i) above is preferably 1.0 or less.

[39] Sn: 0 to 0.50%. Tin (Sn) improves corrosion resistance (initial corrosion resistance) and mechanical strength at high temperatures. Therefore, its content can be adjusted as needed. However, if the Sn content exceeds 0.50%, cracking occurs in the steel plate during sheet production, reducing the toughness of the resulting muffler. Therefore, the Sn content is typically set at 0.50% or less. On the other hand, to achieve the desired effects, the Sn content is preferably 0.01% or more. Considering refining costs and producibility, the Sn content is preferably 0.05% or more, and ideally 0.15% or less.

[40] Al: 0 to 3.0%. Aluminum (Al) is an element that has a deoxidizing effect. Furthermore, Al improves corrosion resistance, as well as mechanical strength at high temperatures and oxidation. Additionally, Al serves as a precipitation site for TiN and a Laves phase, contributing to the fine precipitation of the precipitates and improving low-temperature toughness. Therefore, its inclusion can be adjusted as needed.

[41] However, if the Al content exceeds 3.0%, elongation decreases, leading to reduced weldability and surface quality. Furthermore, coarse Al oxide forms, decreasing low-temperature toughness. Therefore, the Al content is set at 3.0% or less. On the other hand, to achieve the desired effects, the Al content is preferably 0.003% or higher. Considering refining costs, the Al content is preferably 0.01% or more, and preferably 1.0% or less.

[42] Ni: from 0 to 2.0%. Nickel (Ni) is an element that improves toughness and corrosion resistance (initial corrosion resistance) and can therefore be included in the required amount. However, if the Ni content exceeds 2.0%, an austenite phase develops, which reduces formability and the tube's bending capacity. Therefore, the Ni content is typically set at 2.0% or less. Considering production costs, the Ni content is preferably 0.5% or less. Furthermore, the advantageous toughness-enhancing effect of Ni is most pronounced when the Ni content is 0.1% or higher; therefore, the Ni content is preferably 0.1% or higher.

[43] V: from 0 to 1.0%. Vanadium (V) has a corrosion resistance (initial corrosion resistance) and heat resistance enhancement effect when combined with carbon (C) or nitrogen (N). Therefore, its content can be adjusted as needed. However, if the V content exceeds 1.0%, coarse carbonitrides form, reducing toughness. Therefore, the V content is typically set at 1.0% or less. Considering production costs and manufacturability, the V content is preferably 0.2% or less. Alternatively, the V content is preferably 0.05% or more to achieve the desired effect. rcnonn / zznz / E / YiAi

[44] Cu: 0 to 2.0%. Copper (Cu) has a corrosion resistance-enhancing effect (initial corrosion resistance) and a high-temperature mechanical strength-enhancing effect within an intermediate temperature range due to the precipitation of dissolved Cu from the base phase, a phenomenon known as ε-Cu. Therefore, its content can be adjusted as needed. However, excessive Cu content leads to decreased toughness during steel sheet hardening and reduced ductility. For this reason, the Cu content is typically set at 2.0% or less. Furthermore, to achieve the effect described above, the Cu content should preferably be 0.1% or more, and more preferably 1.0% or more. Considering oxidation resistance and producibility, the Cu content is preferably less than 1.5%, and more preferably 1.4% or less.

[45] Mo: 0 to 3.0%. Molybdenum (Mo) is an element that improves corrosion resistance (initial corrosion resistance) and prevents or reduces crevice corrosion, especially in blanks and similar materials with a crevice structure. Therefore, its content can be adjusted as needed. However, if the Mo content exceeds 3.0%, formability deteriorates significantly and producibility decreases. Therefore, the Mo content is typically set at 3.0% or less. On the other hand, to achieve the desired effects, the Mo content is preferably 0.10% or more. Considering alloying costs and productivity, the Mo content is preferably 0.15% or more, and more preferably 2.0% or less. The Mo content is preferably 0.15% or more, and more preferably 0.80% or less.

[46] Ca: from 0 to 0.0030%. Calcium (Ca) is a useful element as a desulfurizer and can therefore be included in the required amount. However, if the Ca content exceeds 0.0030%, coarse CaS is produced, decreasing toughness and corrosion resistance (initial corrosion resistance). Therefore, the Ca content is set at 0.0030% or less. On the other hand, the Ca content is preferably 0.0001% or more to achieve the desulfurizing effect. Considering refining costs and producibility, the Ca content is preferably 0.0003% or more, and preferably 0.0020% or less.

[47] Ga:0 to 0.1%. rcnonn / zznz / E / YiAi Gallium (Ga) may be included as needed to improve corrosion resistance (initial corrosion resistance) and to prevent or reduce hydrogen embrittlement. The Ga content is set at 0.1% or less. However, to achieve the desired effects, the Ga content is preferably 0.0002% or more, taking into account the production of sulfides and hydrides. From the standpoint of production costs and producibility, as well as ductility and toughness, the Ga content is preferably 0.0005% or more, and preferably 0.020% or less.

[48] <Elementos del grupo B> B: 0 to 0.0050%. When segregated at grain boundaries, boron (B) enhances the strength of these boundaries, improving secondary workability and low-temperature toughness. Additionally, B enhances high-temperature strength within an intermediate temperature range. Therefore, its content can be adjusted as needed. However, a B content above 0.0050% leads to the formation of B compounds, such as CcB, which degrade intergranular corrosivity and fatigue properties. Therefore, the B content is typically set at 0.0050% or less.

[49] Furthermore, to obtain the desired effects, the B content is preferably 0.0002% or more. Taking into account weldability and producibility, the B content is preferably 0.0003% or more, and preferably 0.0010% or less.

[50] W:0 to 3.0%. Tungsten (W) has a high-temperature strength-enhancing effect and can therefore be contained as needed. However, excessive W content leads to a deterioration in toughness and a decrease in elongation. Furthermore, it increases the production of a Laves phase, an intermetallic composite phase, which inhibits the development of a {111}-oriented texture. <112> and the r-value decreases. Therefore, the W content is set at 3.0% or less. Considering production costs and producibility, the W content is preferably 2.0% or less. On the other hand, the W content is preferably 0.1% or more to achieve the advantageous effect of improved high-temperature resistance.

[51] Co: 0 to 0.50%. rcnonn / zznz / E / YiAi Cobalt (Co) has a high-temperature strength-enhancing effect and can therefore be contained as needed. However, excessive Co content decreases toughness and workability. Therefore, the Co content is set at 0.50% or less. Furthermore, considering production costs, the Co content is preferably 0.30% or less. On the other hand, to achieve the desired effect, the Co content is preferably 0.02% or more, and more preferably 0.05% or more.

[52] Sb: from 0 to 0.50%. Antimony (Sb) segregates at grain boundaries to increase mechanical strength at high temperatures and can therefore be contained as needed. However, Sb content above 0.50% causes excessive segregation, decreasing the low-temperature toughness of the weld zone of the resulting tube. Therefore, the Sb content is typically set at 0.50% or less. Considering high-temperature properties, production costs, and toughness, the Sb content is preferably 0.30% or less. On the other hand, to achieve the desired effects, the Sb content is preferably 0.01% or more.

[53] <Elementos del grupo C> Mg: 0 to 0.0100% Magnesium (Mg) acts as a deoxidizer by forming Mg oxides in molten steel, similar to aluminum (Al). Furthermore, Mg increases the proportion of equiaxed crystals in the resulting plate, as the finely crystallized Mg oxides serve as nuclei. This eliminates the coarse microstructure of the steel resulting from columnar crystals, which causes surface irregularities, and improves surface quality. Subsequently, the precipitation of fine Nb- and Ti-based precipitates is promoted in a later process. Specifically, when these fine precipitates precipitate during hot rolling, they serve as recrystallization nuclei in both the hot rolling process and a subsequent annealing of the hot-rolled sheet. This results in a very fine recrystallized structure, which contributes to improved toughness.Therefore, it can be contained as needed.

[54] However, excessive Mg content leads to impaired oxidation resistance, decreased weldability, etc. Therefore, the Mg content is set at 0.0100% or less. On the other hand, to achieve the desired effects, the Mg content is preferably 0.0002% or more. Taking refining costs into account, the Mg content is preferably 0.0003% or more, and preferably 0.0020% or less.

[55] Zr 0 to 0.30%. Zirconium (Zr) is an element that improves oxidation resistance and can therefore be included as needed. However, Zr content above 0.30% significantly decreases toughness and manufacturability, as well as pickling properties. Furthermore, Zr compounds with carbon and nitrogen form oils. As a result, the microstructure of the steel sheet becomes coarse-grained during hot rolling and annealing, and the roughness coefficient (r) decreases. Therefore, the Zr content is typically set at 0.30% or less. Considering production costs, the Zr content is preferably 0.20% or less. Alternatively, a Zr content of 0.05% or more is preferable to achieve the desired effect.

[56] Ta: from 0 to 0.10%. Tantalum (Ta) contributes to improved toughness by combining with carbon (C) and nitrogen (N) and can therefore be included in the required amount. However, if the Ta content exceeds 0.10%, production costs increase and producibility decreases considerably. Therefore, the Ta content is typically set at 0.10% or less. On the other hand, to achieve the desired effects, the Ta content is preferably 0.01% or more. Considering refining costs and producibility, the Ta content is preferably 0.02% or more, and preferably 0.08% or less.

[57] REM: from 0 to 0.05%. Rare earth metals (REM) refine various types of precipitates, improving toughness and oxidation resistance. Therefore, their content can be adjusted as needed. However, if the REM content exceeds 0.05%, castability decreases considerably. Therefore, the REM content is typically set at 0.05% or less. On the other hand, to achieve the desired effect, the REM content is preferably 0.001% or more. Considering refining costs and producibility, the REM content is preferably 0.003% or more, and ideally 0.01% or less.

[58] REM (rare earth metal) refers to 2 elements, including scandium (Se) and yttrium (Y) and 15 elements from lanthanum (La) to lutetium (Lu) (lanthanoid), 17 elements in total. rcnonn / zznz / E / YiAi REM content means the total content of these elements, and the elements can be added individually or in the form of a mixture.

[59] In the chemical composition according to the present invention, the remainder consists of Fe and unavoidable impurities. The term unavoidable impurities, as used herein, means components that are mixed into steel during its industrial production due to raw materials such as ores and scrap and various factors in a production process, and are permitted to mix into the steel within their respective ranges where the unavoidable impurities have no adverse effect on the present invention.

[60] 3. Microstructure of steel It is desirable that the microstructure of the base metal steel of the ferritic stainless steel sheet be substantially a single ferritic phase. Specifically, the base metal steel microstructure preferably includes, by volume proportion, 95% or more of a ferritic phase. Note that, for example, an unavoidably produced hard phase, such as a martensitic phase, may be present at 5% or less. The volume proportions of the ferritic and hard phases should be measured using a ferrite gauge, observation of the steel microstructure, and other similar methods.

[61] 4. Nitrided layer The nitrided layer is a layer in which nitrogen is concentrated and which is formed by an annealing nitriding treatment. In the ferritic stainless steel sheet according to the present invention, the nitrided layer refers to a layer present in a region extending from the rolled surface to a depth of 0.05 µm in the direction of the sheet thickness, where the nitrogen concentration is significant. For the ferritic stainless steel sheet according to the present invention, the average nitrogen concentration in the nitrided layer is established at a mass percentage equal to or greater than 0.80%. The average nitrogen concentration in the nitrided layer is preferably 1.0% or more.

[62] The average nitrogen concentration is obtained by measuring a nitrogen distribution in the direction of the sheet thickness by sputtering up to 1 pm from the surface in light discharge optical emission spectrometry (GDS) and calculating an average concentration from the surface of the steel sheet to a position of 0.05 pm. rcnonn / zznz / E / YiAi

[63] The mean nitrogen concentration in the nitrided layer and the initial corrosion resistance will be described here. To evaluate the nitrogen concentrations in the nitrided layers and the initial corrosion resistances, a combined cyclic corrosion test was performed in JASO mode simulating an outdoor corrosive environment (the cyclic corrosion test defined in JASO-M609-92).

[64] Specifically, samples were prepared that were subjected to a nitriding treatment and had different average nitrogen concentrations in their nitrided layers. The average nitrogen concentration was measured using the method described above. The distribution of the nitrogen concentration on a steel sheet surface along its thickness direction is, for example, as illustrated in Figure 1. As can be seen in Figure 1, the nitrogen concentration tends to be higher at the surface and gradually decreases with increasing depth along the thickness direction of the sheet.

[65] As a method for evaluating initial corrosion, the pitting that occurred on a surface of the sample subjected to the cyclic corrosion test was taken as the evaluation portion. Specifically, a test sample was cut to 70 mm x 40 mm, and its end portion was sealed by 5 mm and used as a sample. The cyclic corrosion test was carried out until pitting occurred under test conditions that included: spraying with salt water (5% NaCl) at 35 °C for 2 hours, then drying at 60 °C for 4 hours, and then holding in humid air at 50 °C and a relative humidity of 90% or more for 2 hours, constituting a total process of 8 hours as one cycle. The sample was placed in an apparatus so that it was inclined at 30 degrees with respect to a vertical direction.

[66] Subsequently, the sample was taken out after each cycle, its surface was cleaned, and when no pitting occurred for five or more cycles, the sample was deemed to have sufficient corrosion resistance to prevent initial corrosion from occurring from the time of shipment of a car until before or immediately after use, i.e., initial corrosion resistance, and was rated as passed.

[67] Figure 2 is a graph illustrating the relationship between the average nitrogen concentrations of the nitrided layers and the number of cycles in which pitting occurs. From Figure 2, when the average nitrogen concentration of a nitrided layer is 0.80% or more, a steel sheet is obtained that does not experience pitting for five or more cycles and has excellent resistance to initial corrosion. rcnonn / zznz / E / YiAi

[68] As can be seen from the above, annealing nitriding is useful for improving early corrosion resistance. In this case, nitrogen undergoes active dissolution within a pit in stainless steel at an early stage of pitting. Its dissolution product, NH4+, blocks oxidation inside the pit and promotes the regeneration of a passivation film, thus suppressing pitting and its growth, thereby improving corrosion resistance. However, if nitrogen combines with chromium to form chromium nitride at the grain boundaries, chromium depletion leads to sensitization, and corrosion resistance decreases.Consequently, by performing the annealing nitriding treatment to make a certain amount of nitrogen enter only in the vicinity of a surface of the steel sheet, the N is contained in the surface in a large amount, while the formation of nitride is prevented or reduced, in order to improve corrosion resistance.

[69] 5. Production method A method for producing ferritic stainless steel sheet according to the present invention will be described. The ferritic stainless steel sheet according to the present invention provides its advantageous effects regardless of the production method, provided that the ferritic stainless steel sheet has the configuration described above; however, the ferritic stainless steel sheet can be reliably produced by a production method described below, for example.

[70] 5-1. Continuous plate casting process A preferred method is one in which a steel with the chemical composition described above is melted in a converter and subsequently subjected to secondary refining. The molten steel is then preferably cast into plate according to a known casting method (continuous casting). Note that the casting conditions are adjusted to those of the conventional continuous casting method.

[71] 5-2. Hot rolling process The resulting plate is then preferably subjected to a hot rolling process using continuous rolling to obtain a predetermined sheet thickness. In this case, if the plate heating temperature during hot rolling is below 1100 °C, the alloying elements do not dissolve completely, resulting in precipitates that can adversely affect subsequent processes. On the other hand, if the plate heating temperature exceeds 1250 °C, buckling may occur, where the plate undergoes high-temperature deformation under its own weight. Therefore, it is preferable to set the plate heating temperature during hot rolling between 1100 and 1250 °C. Furthermore, considering productivity and the occurrence of surface defects, the plate heating temperature is even more preferable to be between 1150 and 1200 °C.Note that, in the present invention, the heating temperature of the plate is synonymous with the starting temperature of hot rolling.

[72] In the hot rolling process, it is preferable to subject the heated plate to rough rolling in a plurality of passes and subsequently to finish rolling through a plurality of rolling mills in one direction. In this way, the plate is converted into a hot-rolled sheet and wound into a coil. A final temperature for finish rolling is preferably from 950 to 1150 °C, and a winding temperature is preferably within the range of 600 °C or lower to avoid a decrease in toughness during winding due to the formation of precipitates.

[73] 5-3. Pickling process of hot-rolled sheets For the ferritic stainless steel sheet according to the present invention, it is preferable to subject the hot-rolled steel sheet to a pickling treatment, without annealing the hot-rolled sheet, into a cold-rolling starting material in a cold-rolling process. This differs from the normal production method usually employed, in which the hot-rolled sheet is annealed to obtain a recrystallized structure of a regulated size. Note that annealing the hot-rolled sheet can be performed in a case where, for example, the hot-rolled steel sheet is hard and needs to be softened.

[74] 5-4. Cold rolling process In a cold rolling process, the rolling reduction is preferably 50% or more, and more preferably 60% or more. One reason for setting the rolling reduction within this range is that increasing the rolling reduction increases the stored energy, which serves as the driving force for recrystallization, so that recrystallization can be completed within a temperature range for the annealing nitriding treatment described later.

[75] 5-5. Annealing and nitriding treatment process after cold rolling rcnonn / zznz / E / YiAi In annealing after cold rolling, a steel sheet can be produced in which nitrogen is concentrated on its surface by performing the annealing in a non-oxidizing atmosphere that includes nitrogen gas, with the remainder being hydrogen gas (hereafter simply referred to as annealing nitriding). Although nitriding is generally performed as a separate process after annealing a steel sheet, carrying out the nitriding treatment simultaneously with the annealing of a cold-rolled steel sheet allows for a combination of cost reduction by omitting a step and improved corrosion resistance. For this reason, it is desirable that the annealing and nitriding treatments be performed in a single process.

[76] Here, a nitrided layer formed on a steel sheet surface is formed mainly by the disappearance of a thin passivation film composed of Cr oxides through reduction by hydrogen in the atmosphere and the entry of nitrogen from it under a high temperature atmosphere.

[77] At this time, if nitrogen is scarce, nitriding does not occur sufficiently, and if nitrogen is excessive, reduction by hydrogen does not occur. For this reason, the nitriding gas concentration is preferably within the range of 80 to 99%. The concentration is more preferably within the range of 90 to 98%.

[78] If the nitriding treatment temperature is too low, nitrogen does not enter the process, thus failing to ensure a sufficient amount of nitrogen, and the problem arises of a non-recrystallized structure. For this reason, the treatment temperature is preferably 850 °C or higher. On the other hand, if the treatment temperature is too high, nitrogen may enter in excess. Furthermore, martensite may form in a subsequent process. For this reason, the treatment temperature is preferably 1000 °C or lower. The treatment temperature is most preferably within the range of 880 to 980 °C.

[79] Furthermore, if the treatment duration is too short, insufficient nitrogen penetration occurs, thus failing to ensure a sufficient quantity of nitrogen, and the problem arises of a non-recrystallized structure. For this reason, the treatment duration is preferably 30 seconds or more. On the other hand, the longer the treatment duration, the greater the amount of nitrogen penetrating the surface of the steel sheet, but if the treatment duration is excessively long, nitrogen penetration also occurs in excess. As a result, sensitization occurs due to the formation of nitrides at the grain boundaries, and a martensite phase forms due to phase transformation, leading to a deterioration of corrosion resistance and material quality. For this reason, the treatment duration is preferably 300 seconds or less.The duration of treatment is preferably within the range of 50 to 200 seconds.

[80] To further improve ductility, it is preferable to control the cooling rate after holding at the treatment temperature. If the cooling rate is less than 5 °C / s, nitrides are formed during cooling, leading to sensitization and decreased corrosion resistance. In addition, excess nitrogen may be present, and martensite may form. Likewise, if excessive precipitates form, causing precipitation hardening, ductility decreases. For this reason, the cooling rate is preferably 5 °C / s or higher. On the other hand, if the cooling rate is greater than 100 °C / s, martensite may form, hardening the steel sheet and decreasing ductility. For this reason, the cooling rate is preferably 100 °C / s or lower. The cooling rate is most preferably within the range of 10 to 80 °C / s, and even more preferably within the range of 15 to 50 °C / s.Note that the cooling stop temperature is preferably within the range of 300 to 500 °C.

[81] 5-6. Pickling process after annealing nitriding treatment If scale layers form on the steel sheet after annealing nitriding, the steel sheet shall be pickled as necessary. However, excessive pickling is undesirable because the nitrided layer formed in the process described above dissolves. Therefore, for ferritic stainless steel sheet according to the present invention, in a case where pickling must be performed because scale layers form during annealing nitriding in a non-oxidizing atmosphere, it is necessary to select a pickling condition under which the nitrided layer does not dissolve. Note that a solution and a method for pickling are not limited to a particular solution and method; however, electrolytic pickling is preferred.

[82] 5-7. Other production conditions Other production conditions will be selected as needed. For example, plate thickness, hot-rolled sheet thickness, and similar factors will be adjusted accordingly. In cold rolling, the roll roughness grade (rcnonn / zznz / E / YiAi), rolling oil, number of rolling passes, rolling speed, rolling temperature, etc., will also be selected as appropriate. Additionally, a stress-levelling process can be performed for straightening after annealing, and strip routing can be implemented.

[83] The present invention is described more specifically below by means of examples, although the present invention is not limited to these examples. EXAMPLE

[84] The steels having the chemical compositions shown in Table 1 were melted and then cast into a plate, and the plate was heated to 1150 °C, then hot-rolled to a thickness of 5 mm, and wound at 500 °C into a hot-rolled steel sheet. Note that the chemical compositions at this stage are the chemical compositions of their base metals.

[85] [Table 1] rcnonn / zznz / E / YiAi TABLE 1 Nb+Ti with 014 or Ó 0.14 | 0.14 | 0.30 | 0.50 | 0.23 | ίο b 0.39 | 0.11 | See CXJ 0.94 | θ zero 014 or θ 0.40 | 0.19 | cq — z + o X co 0.09 0.05 0.08 90Ό 0.06 0.05 90Ό 0.06 0.03 90Ό The W3d 1 0.04 . 1 or I- 0.08 ω or N > 1 LO (NI I 1 , 1 ϕ E φ LU 05 2 600Ό 1 I 1 . 1 mo ω 1 1 1 0.44 | Φ 05 O 0.0022 given that o Σ 1 1 1 1 1 1 1 1 is defined as Ó 1 1 1 1 1 1 1 1 cq IIIIII < I 05 cq III ! IIIIII uodsí oles) upo A z 1 1 1 1 1 ! 1 i 1 1 1 ó 1 1 1 cq 1 1 1 1 1 1 1 I ) 1 or π ϕ read Gn < ω 1 1 1 1 1 1 1 |29 1 0.11 1 irvalo c Φ Φ co ó o ó oo 0.20 o O 0.23 CXJ o co o cq ó 0.02 o ó cq o in ó o 0.19 co jn inte %, remainder Z 0.011 0.013 0.017 600'0 0.015 0.012 0.012 0.013 0.015 0.013 900'0 600'0 0.026 0.002 0.013 0.009 0.021 0.012 0.011 0.007 0.013 0.005 0.017 0.011 ^εοΌ I 0.012 | lera de i (masa Ó CXI cq CXI σ> CXI σί 18.0 cq δ 18.0 CO 2 24·7 co cq CXI cri σ> σ> 18.8 σ> cri co CO 22.0 o CO LO eo está íl stal base ω 0.0023 CO o ó 0.0016 co δ o ó 0.0049 0.0016 CXI o ó 0.0013 0.0081 0.0004 0.0049 0.0025 0.0035 co oo ó 0.0016 0.0019 0.0023 0.0042 0.0047 δ o ó 0.0014 0.0010 0.0132* 0.0019 0.0023 I 0.0015 | the brand of the me CL 0.012 CO o ó 0.027 0.029 0.046 0.029 0.042 0.010 0.021 0.021 0.041 0.049 0.049 0.047 0.031 0.044 0.034 090'0 0.033 0.027 LO δ ó 0.013 0.014 0.011 0.040 I 0.031 | lor with oo E OI 0.66 0.33 0.54 co 0.02 0.22 s 66'0 0.54 CO co lo cq cq 0.60 0.38 0.89 cq 0.23 CO 0.54 o cq cq 0.90 co co ó s co OJ > c 3 b uoioisí ω 0.94 cq ΙΛ 0.05 0.96 0.92 0.58 0.65 LO 0.44 ó·42 co 0.66 σ> 0.28 0.82 θ zz-0 CXI 0.92 0.004* Q CXJ 0.53 0.33 cq indicates that E o O 0.018 0.002 CO O 0.010 σ> o 0 δ ó CO oo ó 0.006 0.009 0.009 0.006 CO ooo 0.005 0.025* ooo 0.005 0.006 δ ó 0.007 0.009 | c OJ Steel No. < cq < CO < 3 < < < co < ​​o < < I A11 | CXJ < with I A14 | ιο < A16 < A18 A19 re cq (0 CO 05 ω in CO ce 05 | The sea. rcnonn / zznz / E / YiAi

[86] The pickled hot-rolled steel sheet is then cold-rolled to a 60% reduction using 500 mm diameter rolls and subjected to annealing nitriding treatment by continuous annealing at the temperatures, atmospheres, and durations indicated in Table 2. Note that the cooling rate in the annealing nitriding treatment is 20 °C / s, and cooling was carried out to 350 °C. The annealed sheets thus obtained were each subjected to electrolytic pickling with a 10% aqueous sulfuric acid solution at 60 °C at a current density of 60 A / Dm2 for 10 seconds, on a test sample.

[87] The resulting test specimen was subsequently measured in terms of the volume ratio of its ferritic phase and the average nitrogen concentration in its nitrided layer and then evaluated for corrosion resistance, particularly initial corrosion resistance. In addition, a JIS No. 13B test specimen was cut from the test sample and subjected to a tensile test. Here, the elongations at break of the examples shown in Table 2 were all 20% or more, and the examples were deemed to have no material quality issues.

[88] <Medición de la fase ferrítica> The volume ratio of a ferritic phase was measured using a ferrite gauge. At this point, in a case where the volume ratio did not satisfy the range of the volume ratio of a ferritic phase defined in the present invention, and 5% or more of a martensite phase, which is a phase distinct from ferrite, was produced, it was noted that "Observed on an observation element of a martensite phase" in Table 2.

[89] <Medición de la concentración media de nitrógeno en la capa nitrurada> . The average nitrogen concentration in the nitrided layer was measured on a portion of the steel sheet surface, using light discharge spectrometry (GDS), to determine the nitrogen distribution along the thickness of the sheet. This measurement was performed using sputtering to a depth of 1 pm from the surface of the laminated sheet. The average nitrogen concentration from the steel sheet surface to a depth of 0.05 pm was then calculated as the average nitrogen concentration in the nitrided layer. The GDS measurement conditions were as follows: anode inner diameter: 13 mm, analysis mode: high-frequency mode, discharge power: 30 W, control pressure: 3.5 hPa, and detection wavelength: 110 to 800 nm.

[90] <Evaluación de la resistencia a la corrosión inicial> rcnonn / zznz / E / YiAi To evaluate corrosion resistance, a combined cyclic corrosion test was conducted in JASO mode simulating an outdoor corrosive environment (the cyclic corrosion test defined in JASO-M609-92) to assess initial corrosion resistances.

[91] A specific method for calculating corrosion resistance will now be described. The resulting test specimens were cut to 70 mm × 40 mm, and their end portions were sealed by 5 mm and used as samples. The cyclic corrosion test was carried out until pitting occurred under test conditions that included: spraying with salt water (5% NaCl) at 35°C for 2 hours, then drying at 60°C for 4 hours, and then holding in humid air at 50°C and a relative humidity of 90% or more for 2 hours, constituting a total process of 8 hours as one cycle. Each sample was placed in an apparatus so that it was inclined at 30 degrees with respect to a vertical direction.

[92] Pitting on a sample surface subjected to the cyclic corrosion test was considered part of the assessment of initial corrosion. Specifically, the sample was removed after each cycle, its surface was cleaned, and when pitting did not occur for five or more cycles, the sample was deemed to have sufficient corrosion resistance to prevent initial corrosion from the time a car was shipped until before or immediately after use (initial corrosion resistance), and this was recorded as (O). If pitting occurred within five cycles, the number of cycles after which pitting occurred was recorded in Table 2. The test was carried out for up to seven cycles, and a sample on which no pitting was observed even after seven cycles was considered especially excellent (O).

[93] [Table 2] rcnonn / zznz / E / YiAi TABLE2 Steel Symbol No. Conditions for nitriding annealing treatment of cold-rolled sheets Steel microstructure Average nitrogen concentration in the nitrided layer (%) Property Nitrogen gas concentration (%) Treatment temperature Treatment duration (s) Martensite phase Initial corrosion resistance Example of invention B1 A1 98 950 100 - 0.95 O B2 A2 95 900 100 - 0.83 O B3 A3 98 900 100 - 0.86 o B4 B5 A4 A5 98 98 900 850 100 100 - 0.92 0.81 oo B6 A6 98 950 100 - 0.93 o B7 A7 95 850 100 - 0.84 o B8 A8 98 950 100 - 0.95 or B9 A9 90 980 300 - 1.22 © B10 A10 98 950 100 - 0.92 or B11 A11 90 950 300 - 1.31 © B12 A12 95 950 100 - 0.97 or B13 A13 95 970 100 - 1.20 © B14 A14 98 950 100 - 0.96 or B15 A15 99 950 100 - 0.93 or B16 A16 85 970 100 - 1.28 © B17 A17 90 1000 200 - 1.51 © B18 A18 95 950 100 - 0.98 or B19 A19 95 950 100 - 0.94 or Comparative example b1 a1* 90 950 100 - 0.95 1 b2 a2* 95 900 100 - 0.87 3 b3 a3* 95 900 100 - 0.85 3 b4 a4* 97 900 100 - 0.81 1 b5 a5* 95 900 100 - 0.84 1 b6 a6* 98 900 100 - 0.80 2 b7 a 7* 95 900 100 - 0.82 1 b8 A19 100 950 100 - 0.15* 1 b9 A19 70 950 100 - 0.13* 1 b10 A19 95 1050 100 Observed 3.11 4 b11 A19 95 800 100 - 0.45* 2 b12 A19 95 950 500 Observed 2.36 2 b13 A19 95 950 10 - 0.21 * 3. The mark * indicates that a value with the mark is outside a range of its corresponding element defined in the present invention. The underline indicates that an underlined value is outside its preferred production condition or its objective property in the present invention. rcnonn / zznz / E / YiAi

[94] The symbols B1 to B19 shown in Table 2 each provided a chemical composition that satisfied the ranges defined in the present invention and yielded production conditions that were preferable in the present invention. Therefore, the average nitrogen concentrations of their nitrided layers and their corrosion resistance, specifically the initial corrosion resistance, were also good. Conversely, in one instance of symbols bl to b7, whose compositions were outside the ranges defined in the present invention, their number of cycles in which pitting occurred was insufficient, and therefore their corrosion resistance, specifically the initial corrosion resistance, was poor.Furthermore, in the case of symbols b8 ab 13, whose production methods were outside the preferred ranges in the present invention, the definitions in accordance with the present invention were not met, such as insufficient average nitrogen concentrations of their nitrided layers or the production of a martensite phase, resulting in poor initial corrosion resistance.

[95] In addition, an A19 steel shown in Table 1 was melted and then cast into a plate. The plate was heated to 1150 °C, then hot-rolled to a thickness of 5 mm, and wound at 500 °C into a hot-rolled steel sheet. The pickled hot-rolled steel sheet was then cold-rolled with a 60% reduction using 500 mm diameter rolls and subjected to a nitriding annealing treatment by continuous annealing at the temperatures, atmospheres, durations, and cooling rates indicated in Table 3. The annealed sheets thus obtained were each subjected to electrolytic pickling with a 10% aqueous sulfuric acid solution at 60 °C at a current density of 60 A / Dm² for 10 seconds, on a test specimen.

[96] The resulting test specimen was measured in terms of the average nitrogen concentration in its nitrided layer and ferritic phase using the same procedure shown in Table 2. Regarding properties, the initial corrosion resistance was evaluated using the same procedure shown in Table 2. In addition, a JIS No. 13B test specimen was cut from the test sample and subjected to a tensile test. In the tensile test, an elongation at break of 20% or more was considered sufficient and graded as pass (O), and an elongation at break of less than 20% was graded as fail (x). The results are shown in Table 3.

[97] [Table 3] rcnonn / zznz / E / YiAi rcnonn / zznz / E / YiAi TABLE 3 Steel Symbol No. Conditions for nitriding annealing treatment of cold-rolled sheets Steel microstructure Average nitrogen concentration in the nitrided layer (%) Property Nitrogen gas concentration (%) Treatment temperature (°C) Treatment duration(s) Cooling rate (CC / s) Martensite phase Initial corrosion resistance Elongation Example of invention C1 A19 98 950 100 10 - 0.96 oo C2 A19 95 950 100 45 - 0.94 oo Example c1 A19 90 950 100 200 Observed* 0.92 4 X comparative c2 A19 95 950 100 2 Observed* 2.02 2 X The mark * indicates that a value with the mark is outside a corresponding element range defined in the present invention. The underline indicates that an underlined value is outside its preferred production condition or its objective property in the present invention.

[98] Symbols C1 and C2 each provided a chemical composition that satisfied the ranges defined in the present invention, and their nitrogen gas concentrations, treatment temperatures and treatment durations, as well as the cooling rates in the annealing nitriding treatment, satisfied the respective preferred ranges; therefore, not only their initial corrosion resistances but also their elongations were good. Conversely, symbols 11 and C2 exhibited lower initial corrosion resistance and elongation because their cooling rates did not satisfy the preferred range.

Claims

1. A ferritic stainless steel sheet comprising a base metal and a nitrided layer formed on a surface of the base metal, wherein the chemical composition of the base metal consists, in % by mass, of: C: 0.001 to 0.020%, Si: 0.01 to 1.50%, Mn: 0.01 to 1.50%, P: 0.010 to 0.050%, S: 0.0001 to 0.010%, Cr: 16.0 to 25.0%, N: 0.001 to 0.030%, Ti: 0.01 to 0.30%, Nb: 0 to 0.80%, Sn: 0 to 0.50%, Al: 0 to 3.0%, Ni: 0 to 2.0%, V: 0 to 1.0%, Cu: 0 to 2.0%, Mo: 0 to 3.0%, Ca: 0 to 0.0030%, Ga: 0 to 0.1%, B: 0 to 0.0050%, W: 0 to 3.0%, Co: 0 to 0.50%, Sb: 0 to 0.50%, Mg: 0 to 0.0100%, Zr: 0 to 0.30%, Ta: 0 to 0.10%, and REM: 0 to 0.05%, with the remainder: Fe and unavoidable impurities, rcnonn / zznz / E / YiAi a steel microstructure of the base metal includes, by volume proportion, 95% or more of a ferritic phase, the nitrided layer is a layer that is present in a region ranging from the surface of a laminate to a position of 0.0.5 gm depth in the direction of the sheet thickness, and an average nitrogen concentration in the nitrided layer is, in % by mass, 0.80% or more.

2. The ferritic stainless steel sheet according to claim 1, wherein a chemical composition of the base metal contains one or more elements selected from, in % by mass: Nb: 0.10 to 0.80%, Sn: 0.01 to 0.50%, Al: 0.003 to 3.0%, Ni: 0.1 to 2.0%, V: 0.05 to 1.0%, Cu: 0.1 to 2.0%, Mo: 0.10 to 3.0%, Ca: 0.0001 to 0.0030%, and Ga: 0.0002 to 0.1%.

3. The ferritic stainless steel sheet according to claim 1 or 2, wherein a chemical composition of the base metal contains one or more elements selected from, in % by mass: B: 0.0002 to 0.0050%, W: 0.1 to 3.0%, Co: 0.02 to 0.50%, and Sb: 0.01 to 0.50%.

4. The ferritic stainless steel sheet according to any one of claims 1 to 3, wherein a chemical composition of the base metal contains one or more elements selected from, in % by mass: Mg: 0.0002 to 0.0100%, Zr: 0.05 to 0.30%, pcnonn / zznz / E / YiAi Ta: 0.01 to 0.10%, and REM: 0.001 to 0.05%.